Implemented an InventoryEventProcessor, fixes #1986 (#2176)

* Implemented InventoryEventProcessor, fixes #1986
Event processors can now be registered and unregistered at will. Entity inventory/armor change events are now handled by event processors instead of the inventories themselves, which allows enabling/disabling the calling of these events at will.
This now avoids stupid things happening when initializing inventory contents, since the callers for those events are now registered _after_ the contents are initialized.

src/pocketmine/inventory/InventoryEventProcessor.php

@@ -0,0 +1,48 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\inventory;
+
+use pocketmine\item\Item;
+
+/**
+ * This interface can be used to listen for events on a specific Inventory.
+ *
+ * If you want to listen to changes on an inventory, create a class implementing this interface and implement its
+ * methods, then register it onto the inventory or inventories that you want to receive events for.
+ */
+interface InventoryEventProcessor{
+
+	/**
+	 * Called prior to a slot in the given inventory changing. This is called by inventories that this listener is
+	 * attached to.
+	 *
+	 * @param Inventory $inventory
+	 * @param int       $slot
+	 * @param Item      $oldItem
+	 * @param Item      $newItem
+	 *
+	 * @return Item|null that should be used in place of $newItem, or null if the slot change should not proceed.
+	 */
+	public function onSlotChange(Inventory $inventory, int $slot, Item $oldItem, Item $newItem) : ?Item;
+}